The MCAT is very, very important. The short answer for what score to aim for is "as high as you can get". African-American applicants with a score of 24-26 and a GPA of 3.2-3.39 had a 59% chance of being accepted to at least one medical school. If you can get a 27, your odds go up to 75%. A 30 will bring it to 84%. If you're taking the 2015 MCAT, the scores are different. A score of 24 corresponds to the 39th percentile, while 27 is the 57th. A 30 is 75th percentile.
Do you have any physician shadowing? 20 hours or so would suffice. They want to know that you know what a doctor's day is like. A physician letter is not required: generally, two science professor LORs and one LOR from someone who is not a science professor is required.
Your age will not negatively impact you. As for making your application better, the most important thing by far is a good MCAT score.